This invention relates to a color image forming apparatus, and more particularly to a color image forming apparatus capable of reproducing a color image with high reproducibility irrespective of the kind of an original.
There has been known an image forming apparatus including an image reader and an image recording portion. In the image forming apparatus, an original color image formed on an original (image input medium) such as a color photographic film, television, copy sheet or the like is read out by the image reader to obtain an original image color data for each picture element, the original image color data consisting of data of three color components (red, green and blue) of the color of the picture element. The original image color data is transformed or color-proofed into a printing control data consisting of three colors (cyan, magenta and yellow) printing signals. The printing control data is adapted for controlling the recording portion of the image forming apparatus to record a color image on a recording medium (image output medium) with cyan, magenta and yellow color toners or inks. The color image thus formed on the recording medium has the same colors as the original image formed on the image input medium. As a result, the original image formed on the image input medium is reproduced on the image output medium. The image output medium is, for example, a copy sheet, a television, coated or non-coated sheet, or the like.
In this description, each of the image input medium and the image output medium is defined as a medium for forming or producing a visible color image thereon. The medium is referred to as an "image input medium" when the medium is used as an original having an original color image which is and read by the image reader to be reproduced by the image forming apparatus. The medium is referred to as an "image output medium" when the medium is used as a recording medium on which the original image is to be reproduced.
Color-reproducible ranges for the image input and output media are determined in accordance with both kinds of support material for bearing the image thereon and kinds of image forming systems adapted for forming the image (e.g., kinds of used color materials). Kinds of image input and output media such as, color photographic film, copy sheet, television, printed sheet or the like, are determined in accordance with the kinds of both image bearing support material and image forming systems. Therefore, the color-reproducible ranges of the image input and output media are determined in accordance with the kinds of image input and output media. In the above-described image forming apparatus, inconsistency in color (i.e., color error or color difference) between the original color image on the image input medium and the image reproduced on the image output medium frequently occurs when the color-reproducible range (gamut) of the image input medium is inconsistent with that of the image output medium. For example, a color of the image input medium is recorded on the image output medium with insufficient color-reproducibility, i.e., in the case where the color of the image input medium which is, located inside the gamut of the image input medium, is located outside the gamut of the image output medium.
Various techniques (i.e., color-correcting techniques) for correcting the original image color data have been utilized to correct such a color-inconsistency and improve the color-reproducibility of the original image.
One of the known color-correcting techniques is a color compression technique (i.e., a data-compression transform technique). According to the data-compression transform technique, in a color space, a color point representing the original image color is shifted in a direction toward a predetermined achromtic (neutral) color point to a color-compressed color point which is located within the color-reproducible range of the image output medium. In other words, in the color-compression process, the color point defined by the original image color data which is within the color-reproducible range (gamut) of the image input medium but out of the color-reproducible range (gamut) of the image output medium is shifted (color-compressed) to another color point which is within the gamut of the image output medium.
In this description, the achromatic color (or neutral color) is defined as a color having a lightness, but no hue and chromaticness (saturation). White, gray and black colors belong to the achromatic color, and are decreased in lightness in this order. In the following description, a color axis on which achromatic colors having various lightnesses (white, gray and black) are located, is referred to as the "achromatic color axis". An achromatic color on the achromatic color axis is shifted up to the white color as a lightness thereof is increased, and down to the black color as the lightness is decreased.
Thus obtained data of color-compressed color is then color-proofed to produce the printing control data for driving the recording means to record or reproduce the color-compressed color on the image output medium.
The conventional image forming apparatus, however, has the following disadvantages.
Usually, the image reader installed in the conventional image forming apparatus does not satisfy Luther conditions. In other words, spectral sensitivity of the image reader is different from that of the human visual sensitivity which is represented by color matching functions such as x(.lambda.), y(.lambda.) and z(.lambda.) defined in XYZ colorimetric system, r(.lambda.), g(.lambda.) and b(.lambda.) defined in RGB colorimetric system, and the like which can be converted one to from another through linear conversion. (In this description, the color matching functions such as x(.lambda.), y(.lambda.) and z(.lambda.) are defined as the tristimulus values for human eyes obtained from a monochromatic component of equienergy spectrum.) The original image color data (R, G, B) obtained by the image reader, therefore, fails to represent the human visually sensed color on the original.
It is noted, furthermore, that spectral reflection characteristics of color materials used in the originals are different from one another, in the case where the kinds of the originals, i.e., the kinds of the used color materials are different from one another. The original image color data (R, G, B) is obtained dependently both on the spectral sensitivity of the image reader and the spectral reflection characteristics of the color materials on the originals. 0n the other hand, the human visual sensed color, represented by the tristimulus values (X,Y,Z), is determined dependently on the spectral sensitivity of the human eyes represented by the color matching functions x(.lambda.), y(.lambda.), z(.lambda.) and the spectral reflection characteristics of the color materials on the originals.
Accordingly, such a case may occur that although the colors on different kinds of originals are sensed by human eyes as equal to one another, the original image color data obtained therefor have values different from one another. Another case may further occur that although the colors on the different kinds of originals are sensed by human eyes to be different from one another, the original image color data obtained therefor have the same values.
According to the conventional image forming apparatus, the color-correcting operation is applied to the original image color data obtained through the image reader, as described above. Through the color-correcting operation, the printing control data corresponding to the original image color data is obtained. Since the color-correcting operation is not applied to the human visually sensed true color on the originals, it is impossible to obtain such a printing control data that properly corresponds to the visually sensed color and therefore it is impossible to reproduce the colors on the originals with sufficient reproducibility. For example, in the case where colors on the different kinds of originals are visually sensed to be equal to one another but the original image color data obtained therefor are different from one another, the printing control data obtained for the original image color data are different from one another, so that the colors reproduced in accordance with the printing control data are made different from one another. Furthermore, in the case where colors on the different kinds of originals are visually sensed to be different from one another but the same original image color data are obtained therefor, the same printing control data are obtained, and the same colors are reproduced in accordance therewith.
As is apparent from the above, according to the conventional image forming apparatus, it is impossible to obtain the printing control data properly corresponding to the visually sensed color states of the colors on the originals. Therefore, it is impossible to reproduce the colors on the originals with sufficient reproducibility, in a human visual sense. In addition, the reproducibility largely varies dependently on the kinds of the originals, i.e., the kinds of the color materials. Therefore, it is impossible to reproduce the colors on the different kinds of originals with constant high reproducibility.